Place chicken breasts in a slow cooker and top with butter and dry Italian dressing mix.
Cover and cook on low for 5 to 6 hours or until chicken is tender.
In a small saucepan over medium heat, combine cream cheese and condensed cream of chicken soup. Stir until smooth.
Add sliced mushrooms and mix well.
Pour the sauce over the cooked chicken in the slow cooker.
Cover and cook on low for an additional 1 hour.
Serve over egg noodles or rice.